Barrier coatings which prevent, reduce, or inhibit the permeation of a selected substrate with a gas, vapor, chemical and/or aroma have been widely described, and such coatings are used in a variety of industries, e.g., the packaging industries, automobile industries, paint industries, tire industries etc.
It is well known that the barrier properties of a polymer can be improved by the addition of impermeable plate like structures. When the plates are oriented perpendicular to the diffusion (permeation) direction, the diffusing molecules must go around the plates. This leads to significant reductions in the permeability of the polymer. See, for example, E. L. Cussler et al, J. Membrane Sci. 38:161–174 (1988); W. J. Ward et al, J. Membrane Sci., 55:173–180 (1991); Chang, J. et al, Journal of Applied Polymer Science, Vol. 84, 2294 (2002); Yano, K. et at, Journal of Polymer Science A: Polymer Chemistry, 35, 2289 (1997); Lan, T. et al, Chem. Mater. 6, 573 (1994); Messersmith, P. B. and Giannelis, E. P, Journal of Polymer Science A: Polymer Chemistry 33,1047 (1995); U.S. Pat. Nos. 4,528,235; 4,536,425; 4,911,218; 4,960,639; 4,983,432; 5,091,467; and 5,049,609; and International Patent Application No. WO93/04118, published Mar. 4, 1993, among others.
Despite the numerous disclosures of barrier coatings mixtures, most of the coatings useful in the industry either do not optimally reduce permeability or tend to be brittle and non-flexible. For example, attempts to improve the gas permeability of butyl rubber as well as retain its elasticity and fatigue resistance, have involved coating butyl rubber in tires with a polymer containing a platelet filler. See, e.g., U.S. Pat. Nos. 4,911,218 and 5,049,609. Only minimal decreases in permeability were achieved by this process.
Other attempts to improve the gas barrier properties of rubber used in tires have included compositions of rubber having layered silicate platelets dispersed within the rubber composition. Se,e e.g. U.S. Pat. No. 4,857,397; WO97/00910 and G. J. van Amerogen, “Diffusion in Elastomers”, Rubber Chem Tech, 37, pp 1065–1152 (1964).
Most of the earlier work uses relatively low aspect ratio platelets, at low concentrations, and thermoplastically processed at high shear rates. These conditions lead to relatively small improvements in the barrier properties of the polymer. This is because the reduction in permeability varies rapidly with the aspect ratio and the concentration of plates when the plates are well aligned [E. L. Cussler et al, J. Membrane Sci. 38:161–174 (1988); L. E. Nielsen, Journal of Macromolecular Science, Chemistry AI, 929,(1967); R. K. Bharadwaj, “Modeling the Barrier Properties of Polymer-Layered Silicate Nanocomposites”, Macromolecules 34, 9189 (2001); G. H. Fredrickson and J. Bicerano, “Barrier properties of oriented disk composites”, Journal of Chemical Physics 110, 2181 (1999)]. If the plates are not well aligned, the reductions in permeability are further reduced. The targeted application of these earlier efforts was not coatings, but a bulk polymer with improved barrier and/or mechanical properties.
The use of platelet fillers in coating formulations is also well known. Most often, they have been used in paints to modify the rheology, enabling the production of no-drip paints. These platelet fillers are typically montmorillonites or other exfoliated silicates with aspect ratio of 50 or less. They form a house of cards type structure in the coating suspension that gives a gel like property to the paint (or coating) when it is not undergoing any shear. These structures do not have the plates aligned properly to significantly reduce the permeability of the coating.
The use of exfoliated silicates to produce nanocomposite barrier coatings has been achieved by several methods. The most widely used has been by combining a dissolved polymer with exfoliated filler. Water soluble polymers such as polyvinyl alcohol (PVOH) have been combined with water exfoliated filler such as vermiculite [Japan patent 11-246729, Sep. 14, 1999, “Gas-Barrier Poly(vinyl alcohol)/poly (acrylic acid) Compositions and their Laminates and Shaped Articles”. Sumitomo Chemical Co., Ltd. Polycarbonate dissolved in toluene has been combined with organically functionalized filler to form good barrier coatings [W. J. Ward et al, “Gas Barrier Improvement Using Vermiculite and Mica in Polymer Films”, Journal of Membrane Science, 55:173–180 (1991)]. Other polymers have also been made into improved barrier coatings by dissolving them in a solvent, and using an organically functionalized filler to improve the barrier properties [Yano, K. et al, “Synthesis and Properties of Polyimide-Filler Hybrid Composites”, Journal of Polymer Science A: Polymer Chemistry, 35, 2289 (1997)].
An alternative method that has been used to form nanocomposites has been to incorporate the exfoliated filler into the monomer before polymerization [U.S. Pat. No. 4,472,538 “Composite Material Composed of Filler Mineral and Organic High Polymer and Method for Producing the Same”, Sep. 18, 1984; U.S. Pat. No. 4,889,885 “Composite Material Containing a Layered Silicate”, Dec. 26, 1989]. In some cases, this has been done in aqueous dispersion. Several monomers that can be polymerized into elastomers had exfoliated filler incorporated into the monomer droplets before the emulsion polymerization [PCT Patent No. WO 97/00910, Jan. 9, 1997, “Polymer Nanocomposite Formation by emulsion Synthesis”, Exxon Research and Engineering Co]. Methacrylate monomer was combined with exfoliated filler in aqueous dispersion prior to its polymerization into a nanocomposite [Lee, D. C. and Jang, L. W., Journal of Applied Polymer Science, Vol. 61, 1117–1122 (1996)]. None of these methods led to practical coating formulations. They were designed to help make bulk nanocomposites for thermal processing.
Several references have been made to the orientation of platelet materials in rubber and polymeric compositions. Specific perpendicular orientation of the platelets to the direction of gas diffusion has been found to decrease gas permeability of rubber compositions containing layered silicate platelets, while not adversely affecting the flexibility of the rubber. See, e.g. U.S. Pat. Nos. 5,576,372; 5,576,373; and 5,665,183. Puncture resistance is increased in polymeric sheet material comprising discrete platelets, which are oriented substantially parallel to the plane of the sheet material in an overlapping interrelation. See, e.g., U.S. Pat. No. 5,665,810.
Most of the coatings useful in the industry which contain platelet type fillers are prepared by melt processing, in which solid polymer and solid filler are melted together and mixed at high shear rates. Such melt-processed coatings have 100% solids, and usually use less than about 3% by weight of the platelet fillers. Such coatings do not optimally reduce permeability.
Tires with integral innerliners are disclosed in U.S. Pat. No. 5,178,702, wherein the tire has a top layer and multiple layers of rubber laminate in which at least two layers are barrier layers comprising a sulfur cured rubber composition having 100 parts by weight rubber, 100 parts by weight acrylonitrile/diene polymer and about 25–150 parts by weight of platy filler of unspecified width and thickness. These compositions are stated to reduce the costs of the innerliners while maintaining flexibility and barrier performance.
There are several examples of using an aqueous dispersion of exfoliated filler with an aqueous dispersion of polymer to form a nanocomposite. Most of that work used elastomeric polymers in suspension [Wu, Y—P et al, “Structure of Carboxylated Acrylonitrile-Butadiene Rubber (CNBR)-Filler Nanocomposites by Co-coagulating Rubber Latex and Filler Aqueous Suspension”, Journal of Applied Polymer Science, 82, 2842–2848 (2001); Wu, Y—P et al, “Structure and Properties of Nitrile Rubber (NBR)-Filler Nanocomposites by Co-coagulating NBR Latex and Filler Aqueous Suspension”, Journal of Applied Polymer Science, 89, 3855–3858 (2003): Varghese and Karger-Kocsis, “Natural rubber-based nanocomposites by latex compounding with layered silicates”, Polymer (in press) (2003); Feeney et al, U.S. Pat. No. 6,087,016, “Barrier Coating of an Elastomer and a Dispersed Layered Filler in a Liquid Carrier”, Jul. 11, 2000; Feeney et al, U.S. Pat. No. 6,232,389, “Barrier Coating of an Elastomer and a Dispersed Layered Filler in a Liquid Carrier and Coated Articles”, May 15, 2001; Goldberg et al, “Nanocomposite Barrier Coatings for Elastomeric Applications”, Materials Research Society, Symposium T: Polymer nanocomposites, paper T4.7, (April 2002); Goldberg et al, “Elastomeric Barrier Coatings for Sporting Goods”, ACS Rubber Section, Apr. 29, 2002, paper 17, published in Rubber World, vol. 226, No. 5, p 15 (August 2002)]. The typical use of ion exchange to make the filler surface more compatible with the polymer is not used in these references, in that it usually makes the filler fall out of aqueous suspension. In order to form a nanocomposite from a combination of polymer spheres and filler platelets, one needs significant flow and deformation of the polymer.
None of the above art discusses treatments of the filler that enhance the flexibility of the final nanocomposite.
There remains a need for low permeability coatings with improved flexibility in a variety of applications. These applications include chemical protective gloves, face masks and protective suits. Other uses include low permeability coatings for sports balls, rubber hoses, inflatable boats and other inflatable products, bladders used in production and to protect storage tanks, window sealing, inner tubes for bicycles, and tires. Pneumatic products need improved air barrier. Rubber hoses for fuel lines, protective equipment (such as gloves) and other products need improved resistance to petroleum oils and gasoline. Rubber is often used as protection against corrosion (storage tanks) and hazardous chemicals (gloves, chemical protective suits, face masks and hoods).